Comprehensive Notes – External Respiration & Gas Transport

Distinguishing Internal vs. External Respiration

• Two distinct meanings for “respiration”
  • Internal (cellular) respiration = oxidative breakdown of metabolites to release energy; by-products $CO<em>2$ and $H</em>2O$.
  • External (organismic) respiration = physical gas exchange with environment; supplies $O<em>2$, removes $CO</em>2$.
• This chapter focuses exclusively on the second meaning – the mechanics and control of gas exchange between organism and environment.

External Respiration in Small & Simple Organisms

• Unicellular & very small multicellular organisms
  • Gas moves solely by diffusion across plasma membrane.
  • Membrane must remain moist; desiccation throttles diffusion.
  • Driving force = partial-pressure gradients of $O<em>2$ and $CO</em>2$ in surrounding medium.
• Diffusion constraints on size
  • Surface area grows as $r^2$ whereas volume & metabolic demand grow as $r^3$.
  • Diffusion time increases with distance; sets an upper limit to cell diameter.
• Flat multicellular forms (e.g., flatworms) keep every cell near surface, so diffusion still suffices.
• As body plans thicken/complexify, interior cells are farther from environment; supplementary strategies become essential.

Adaptations That Supplement Diffusion

• General rule: Diffusion handles the final micrometers; bulk flow systems move gases quickly over greater distances.
• Metabolic level matters
  • Invertebrates typically have lower metabolic rates than homeothermic vertebrates, so diffusion needs are less extreme but still significant.

Circulatory-Based Gas Dissemination

• Seen from annelids upward.
  • Blood or other circulatory fluids carry $O<em>2$ inward & $CO</em>2$ outward (bulk flow).
  • Respiratory pigments (hemoglobin, hemocyanin) enhance $O_2$ capacity.

Tracheal Systems of Insects & Arachnids

• Structure
  • Spiracles (surface openings with valves) → large tracheae (chitin-reinforced) → finer tracheoles that end next to individual cells.
• Function
  • Air delivered almost directly to tissues; little reliance on hemolymph for gas transport.
  • Analogous to stomata + intercellular air spaces in a plant leaf.
• Size limitation: Diffusion distance inside tracheoles limits insect body size.

Aquatic Gills

• General design: Thin, moist, highly vascularized epithelial extensions that maximize surface area.
• Bony fishes (teleosts)
  • Five gill arches per side, each bearing feathery filaments → further subdivided into lamellae to enlarge area.
  • Protected by operculum.
  • Water enters mouth, exits through gill slits; countercurrent flow between blood & water maximizes gradient.
• Variants
  • Simple naked flaps in many invertebrates.
  • Internal pharyngeal gills in some forms; gill basket in amphioxus & tunicates doubles as filter-feeding apparatus (suggesting evolutionary origin).
• Ventilation strategies
  • Primitive fishes: rely on continuous forward swimming; can drown if immobilized.
  • Most teleosts: perform active buccal-opercular pumping with valves & cavity-volume changes (Fig. 20.3).
• Air-gulping fishes: swallow atmospheric air; absorb $O_2$ across vascular oral lining – transitional step toward lungs.

Mammalian Respiratory Anatomy (Human Model)

• Air conduction pathway
  • Nose/mouth → nasal pharynx (air warmed & filtered, more efficient nasally) →
  • Pharynx → Glottis (tracheal opening) guarded by epiglottis →
  • Larynx (voice box) → Trachea (cartilaginous rings) →
  • Primary bronchi (left & right) → successive bronchioles →
  • Terminal alveolar sacs (~300 million in human lungs).
• Structural notes
  • Palate separates nasal & oral cavities (hard anterior, soft posterior).
  • Bronchioles lack cartilage; subject to inflammation (bronchitis).

Gas Exchange at the Alveoli

• Alveoli: thin-walled sacs richly coated with pulmonary capillaries.
• Partial-pressure relationships (Example 1)
  • Inspired air $O<em>2$ fraction = $20.96\%$ → $pO</em>2 \approx 160\,\text{mm Hg}$.
  • Expired air $O<em>2$ fraction = $15.8\%$ → $pO</em>2 \approx 120\,\text{mm Hg}$.
  • ~$25\%$ of incoming $O_2$ extracted per breath.
  • $CO_2$ rises from $0.03$–$0.05\%$ (inspired) to $4\%$ (expired); tension ↑ ≈ $100\times$.
• Diffusion gradients
  • High alveolar $pO<em>2$ → $O</em>2$ diffuses into blood.
  • High blood $pCO<em>2$ → $CO</em>2$ diffuses into alveoli for exhalation.
• Comparative surface area: Mammalian lungs have vast alveolar area vs. smooth, sac-like amphibian lungs; reptile lungs intermediate (spongy but not alveolated).

Mechanics of Mammalian Breathing

• Musculature & cavities
  • Diaphragm (unique to mammals) = dome-shaped muscle forming pleural cavity floor.
  • External intercostals lift rib cage.
• Inhalation (active)
  • ↑$CO_2$ in blood → medullary centers send impulses down phrenic nerve.
  • Diaphragm contracts (flattens), intercostals contract → pleural volume ↑, intrathoracic pressure ↓ → air sucked in.
• Exhalation (normally passive)
  • Phrenic firing stops (blood $CO_2$ now lower).
  • Diaphragm & intercostals relax → cavity volume ↓, pressure ↑ → air expelled.
  • Yellow elastic connective tissue in lungs recoils, aiding expiration.

Lung Volumes & Capacities

• Tidal volume ≈ $500\,\text{mL}$ (quiet breath).
• Inspiratory reserve ≈ $3\,\text{L}$ (extra in after tidal inspiration).
• Expiratory reserve ≈ $1\,\text{L}$ (force-out after tidal expiration).
• Vital capacity = tidal + inspiratory reserve + expiratory reserve ≈ $4.5$–$5\,\text{L}$.
• Residual volume > $1\,\text{L}$ even after maximal expiration; keeps alveoli open & gas exchange continuous between breaths.

Neural & Chemical Regulation of Breathing

• Central pattern generators in medulla oblongata & pons
  • Inspiratory centers: rhythmic 2 s bursts → phrenic & intercostal nerves.
  • Expiratory centers: mostly silent at rest; activate during forced breathing.
• Reflexes
  • Stretch receptors in bronchi/bronchioles → via vagus to expiratory centers → inhibit inspiration (Hering–Breuer reflex).
  • Pneumotaxic center (pons) provides additional inspiratory cut-off.
• Chemoreceptors
  • Peripheral: carotid bodies & aortic arch; sense $pH$, $pCO<em>2$, $pO</em>2$.
  • Central: medullary CSF sensors sensitive to $pH$ shifts.
  • Primary driver: rising $pCO<em>2$ / falling $pH$ (hydrogen ion concentration) > falling $pO</em>2$.

Transport & Exchange of $CO<em>2$ and $O</em>2$ in Blood

• Fates of tissue-derived $CO_2$
  1. ~$10\%$ dissolves in plasma.
  2. Some binds globin amino groups forming carbaminohemoglobin (HbNHCOOH).
  3. Majority reacts in erythrocytes:
     $CO<em>2 + H</em>2O \xleftrightarrow{\text{carbonic anhydrase}} H<em>2CO</em>3 \xleftrightarrow{} H^+ + HCO_3^-$
• Bicarbonate handling
  • $HCO_3^-$ diffuses into plasma; chloride shift moves $Cl^-$ into RBCs to maintain electroneutrality.
• At lungs
  • Low alveolar $CO<em>2$ reverses reactions; carbaminohemoglobin ++ $HCO</em>3^-$ convert back to free $CO_2$ which diffuses out.
• Hemoglobin dynamics
  • Bohr effect: high $CO<em>2$ / low $pH$ in tissues ↓ Hb affinity for $O</em>2$, aiding unloading.
  • Cooperativity: each bound $O_2$ increases affinity of remaining hemes → rapid uptake in lungs; yields sigmoid oxygen-saturation curve.

Comparative & Applied Examples (Solved Problems)

Earthworm vs. Insect (Problem 20.1)

• Earthworm
  • Cutaneous respiration through moist skin; slow diffusion requires a circulatory system to distribute gases internally.
• Insect
  • Tracheal system delivers air directly; therefore less dependence on circulatory transport. Effectiveness limits maximum body size.

Avian Respiratory Efficiency (Problem 20.2)

• Extra components: anterior & posterior air sacs extending even into hollow bones.
• Unidirectional air flow
  1. Inhalation: fresh air → posterior sacs; previous lung air → anterior sacs.
  2. Exhalation: posterior sacs → lungs; anterior sacs → outside.
• Consequences
  • Near-complete air turnover (no “dead-space” stagnation as in mammalian blind sacs).
  • Countercurrent exchange: blood flows opposite to air across parabronchi → maximizes gradient.
  • Flight muscles & wing motion act as bellows; birds lack diaphragm.

Marine Mammal Diving Adaptations (Problem 20.3)

• Physiological traits
  • Blood volume ≈ $2\times$ terrestrial mammals; larger vessels & high RBC count.
  • Elevated myoglobin in muscle for extra $O_2$ storage.
  • Ability to bradycardia (slow heart) & peripheral vasoconstriction; prioritize brain & heart.
  • Greater reliance on anaerobic metabolism during extended dives.
  • Often exhale before diving → reduced buoyancy & decompression risk.

Hemoglobin Cooperativity & Sigmoid Curve (Problem 20.4)

• Without cooperativity, $O<em>2$ binding would be linear vs. $pO</em>2$.
• Binding of first $O_2$ changes heme conformation → increases affinity of remaining sites → produces sigmoidal (S-shaped) saturation curve, crucial for both efficient loading (lungs) & unloading (tissues).